Scratch-off tickets used in instant lottery are typical of sheets having an opacifying or hiding layer which hides images printed beneath and which is readily removable as by scratching or rubbing. Such scratch-off tickets are described in JP-U 2-3897 and JP-A 2-297482.
The scratch-off ticket is constructed such that a printed sheet based on a paper or similar substrate is locally covered with a hiding layer so that the printed images are hidden thereby. The hiding layer can be readily removed by scratching it with a nail, coin or pick, or by attaching adhesive tape thereto and peeling the tape.
The hiding layers are conventionally formed from compositions comprising inorganic powder opacifiers such as aluminum fine powder, brass fine powder, copper fine powder and such metal oxide fine powder, synthetic resin vehicles such as acrylic resins and polyester resins, and organic solvents for dissolving them.
Using suitable techniques such as printing, these compositions are coated to substrates having printed thereon images to be hidden, to form hiding layers thereon. The synthetic resins commonly used as the vehicle are relatively hard resins for the reason that when an amount of heat just enough for drying or solvent evaporation is applied, the resin is converted to an under-cured state because of its relatively slow cure rate and thus retains a certain degree of binder function. More energy and time are necessary for the resin to fully cure, which is unwanted from the standpoint of manufacturing process. It would be desirable to have more curable vehicles.
Under the current circumstances, hard vehicles must be used. However, hard hiding layers adhere poorly to flexible substrates of paper or plastic film, failing to provide a satisfactory tack or temporary bond. It is then a common practice to form an intercoat between the substrate and the hiding layer for improving the temporary bonding force. It is then necessary to form two coats, intercoat and hiding layer, and the overall manufacturing process becomes a cumbersome operation.
Since the intercoat for improving the temporary bonding force is formed of a material which is well adherent to the hiding layer, it interferes with the ease of removal of the hiding layer by scratching, indicating that the necessary function of the hiding layer is lost. A countermeasure taken against this interference is to add a silicone parting agent to the hiding layer composition. Small amounts of the parting agent fail to ensure ease of scratch-off whereas large amounts of the parting agent detract from the temporary bonding force. It is then difficult to determine an optimum amount of the parting agent. An increased cost is also a problem.